Update a shapekey Basis from another shapekey
Let's say you have 3 shapekeys: Base, new Face, expression.
The first is the old mesh, the second the new modified mesh, the third the old expression movement you want to preserve.
Select the object and the basis mesh, enter edit mode, select all vertices, Ctrl V "Blend from shape": in the toolshelf (T) in the bottom left of the 3D view you will have to choose the "new Face" shape key, and set an amount (Typically 1.0).
Exit edit mode and you should have your expression shape key working on the new face.

Once you understand the principle you will have to tweak the values and the shape keys, maybe propagating not all but only some vertices, setting the amount not to fully 1.0, or tweaking your old expression shape keys, but you will have a good starting point.